This invention is concerned generally with switching power supplies and, more particularly, with a self-commutated SCR power supply that can supply maximum power over a wide range of output voltages and currents to a wide variety of load impedances, including loads having high reactive impedance components.
Switching power supplies, or D.C. to D.C. converters, have traditionally combined high efficiency with small size. One typical configuration of the prior art is the transformer coupled power supply having an A.C. input voltage that is rectified. The resulting D.C. voltage is applied to a switching device such as a transistor or an SCR for opening and closing the circuit to create an A.C. current through the primary of the transformer. The output of the secondary of the transformer is then rectified and filtered, using one or more stages of filtering with inductors and capacitors in a well known manner. Regulation is achieved by using an error signal from the output voltage, in constant voltage operation, or from the output current, in constant current operation, to control the duty cycle or frequency, or both, of the switching device. In these power supplies, it has been a problem to obtain both low ripple on the output power signal, and feedback stability when the power supply is operating into a load with a high reactive impedance component. The switching device is typically turning on and off under high current conditions resulting in high power losses in the switching device and inducing square wave power signals in the secondary of the transformer which are hard to filter out. The dissipation and drive requirements of currently available economical transistors for the switching device limits the practical power supply output power. SCR's are thus more desirable at the higher power levels but high (di/dt) through the SCR during switching and the need for extra SCR commutating circuitry reduces the usefulness of SCR's for the switching devices as well.
Various techniques were attempted in the prior art to extinguish the conduction of SCR switches and to control the power supplied to the load. Since SCR's require a fixed minimum turn-off time during which a forward bias voltage cannot be reapplied to the SCR without it becoming again conductive, various techniques of either forced reverse biasing of the SCR or resonating the SCR current to zero for a period at least as long as the required minimum SCR turn-off time were devised.
The Morgan circuit is a well known direct coupled SCR chopper inverter circuit that is single ended. In this circuit the SCR is self-commutated by an LC circuit which includes a saturable reactor. When the reactor saturates, the SCR rings off with high switching losses and RFI since the SCR is carrying a full load current during ringing off. Also, load energy limits are not provided by this circuit so that for a large load current the LC resonant circuit will not have enough energy to ring off the SCR, and, in order to vary the power level of the D.C. output power signal, this circuit requires that the SCR trigger frequency vary for different loads. This type of power output control is called a time-ratio control, which necessitates operating the SCR in the audible frequency range to vary the power supply output signal over a useful range.
Another SCR power circuit of the prior art is a voltage doubler with a fixed output voltage as discussed in the paper by J. A. Pirraglia and R. Kande entitled "A 15 KC-DC to DC Converter,"and printed in the IEEE Conv. Record of the Industrial Static Power Converter Conf., No. 4, Nov. 1965, pp.2224-233. This circuit employs the natural resonance of two capacitor coupled loops as the means of energy transfer and SCR commutation. The current through the SCR's being nearly sinusoidal provides for low initial di/dt and switching losses of each SCR. Also, diode clamping of a capacitor common to both loops limits the energy that may be transferred through the resonant loops. Without the diode clamp the voltage on the unclamped capacitor would attain an uncontrolled level. Additionally, no capability for varying the output power is provided in this circuit; however, by varying the frequency of the SCR trigger pulses, the output power can be varied, which necessitates operating the SCR in the audible frequency range, and creates variable frequency RFI which is difficult to eliminate with fixed filters.
Yet another type of self-commutated power supply is described by Patrick W. Clarke in a paper entitled "Self-Commutated Thyristor D.C. to D.C. Converter," and presented at the IEEE Workshop on Applied Magnetics, Washington, D.C., May 22-23, 1969. In this circuit an LC resonant circuit is used to commutate a pair of SCR's. The inductances of this resonant circuit are contributed by the primary windings of a coupling transformer which also isolates the load from the input circuit. The voltage on the output capacitor is reflected back to the input side of the transformer and causes each of the SCR's in turn to abruptly clamp off, resulting in high di/dt switching losses, and RFI on the output signal. By providing a minimum load across the output capacitor, the voltage across the capacitor is prevented from charging to greater than twice the input voltage. Without this minimum load, the output energy is limited only by the Q of the circuit. In this circuit, like those described above, the output power can only be varied by a time-ratio control technique for triggering each of the SCR's.
As mentioned previously, the prior art power supplies typically regulate the output signal by feeding back an error signal from either the output voltage when in the constant voltage mode, or the output current when in the constant current mode to control the duty cycle or frequency of operation, or both, of the switching device. The stability of these control loops becomes a problem when a large reactive load is placed on the power supply. A power supply in the constant current mode loaded with an inductor or in the constant voltage mode loaded with a capacitor results in the inclusion of an additional pole in the respective feedback loop, causing an additional phase shift that can cause the control loop to oscillate. It is well known that if a circuit with a loop gain of greater than unity experiences a phase shift of 180.degree. in the feedback loop, it will oscillate, thus, the greater the reactive impedance of the load, the greater the possibility of oscillation. To prevent power supply oscillation, the feedback loops of the prior art power supplies were either unable to tolerate loads with large reactive impedance components or were tailored to the specific load to prevent oscillations.